The use of blow molded thermoplastic containers and other hollow articles has become commercially significant as disclosed, for example, in the article "Blow Molding: The Next Five Years," Plastics Technology, (June, 1979), pages 61-64. Blow molding is a process which makes possible construction of intricately shaped, lightweight, corrosion-resistant containers, which have high mechanical strength. Containers made from thermoplastic resins can be used for the storage of aqueous or highly polar liquids and, for this purpose, are essentially impervious to the substances stored therein. However, blow molded thermoplastic containers are not entirely satisfactory for the storage of relatively nonpolar organic liquids because the organic liquids can diffuse through the walls of the thermoplastic container at an unacceptably high rate.
It would be highly desirable to be able to use blow molded thermoplastic containers for safe and long-term storage of commercially significant nonpolar fluids, including gasoline and other liquid fuels, motor oils, hydrocarbon-based cleaning fluids or household solvents and oil-based paints containing hydrocarbon solvents. In presently available thermoplastic containers, diffusion of hydrocarbon solvents through the walls thereof often leads to an unacceptable loss of at least part of the solvent material contained therein. As a result, the properties of the stored materials, for example, oil-based paint, may change so drastically as to become useless. It will also be apparent that blow molded containers for hydrocarbon fluids, e.g. gasoline tanks, have met with marginal commercial acceptance owing to the loss of fuel therefrom. Diffusion of even small amounts of gasoline through the walls of a fuel container will contribute to air pollution.
In addition to the use of thermoplastic containers for hydrocarbon liquids, thermoplastic materials can be used to package a variety of other commercially significant materials. Polyethylene terephthalate (PET) is widely used in containers for carbonated beverages. Aqueous ammonia solutions and hypochlorite bleach solutions are commonly packaged in thermoplastic bottles. Oxygen-sensitive materials, such as meats, are packaged in thermoplastic films. It is contemplated that dilute acetic acid, i.e. vinegar, could be packaged in plastic materials, as well as home permanent waving solutions.
Thermoplastic resins which can be blow molded include polymers and copolymers of styrene, acrylonitrile, vinyl chloride and olefins containing at least one aliphatic mono-1-olefin having a maximum of 8 carbon atoms. Polyethylene terephthalate is representative of a polyester type condensation polymer which can be blow molded. The preferred types of materials for blow molded containers are, however, polyolefins, that is, homopolymers and copolymers of ethylene, propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 3-methyl-1-butene, 3,3-dimethyl-1-butene and the like.
Attempts to overcome the tendency of nonpolar organic fluids to diffuse through the walls of blow molded thermoplastic containers have included treating the surfaces of the containers both during and after the blow molding process, as well as the use of polymer blends.
One representative post-treatment method for providing a barrier layer on the surface of a polyolefin object and making blow molded polyolefin containers relatively impermeable to nonpolar solvents has been proposed by Joffre in U.S. Pat. No. 2,811,468. In this process, the internal surface of a blow molded bottle is fluorinated with pure fluorine or with a mixture of fluorine and air/nitrogen. The fluorinated containers thus produced have much better barrier properties toward hydrocarbon solvents then untreated containers. The barrier properties were determined by testing with allyl caproate, a volatile, highly odoriferous material. However, even accelerated testing requires a long period of time.
A more effective and economical way of obtaining blow molded containers, having enhanced barrier properties to hydrocarbon solvents, is proposed by Dixon et al. in U.S. Pat. No. 3,862,284. Dixon et al. teach that, in the blow molding of thermoplastic materials, 0.1-10% by volume of fluorine and 99.9-90% by volume of an inert gas are blended into a fluid medium before expanding the parison of the container to the contour of the mold. Containers produced by this process, using the AIROPAK.RTM. system, have an interior surface which is extremely resistant to permeation by nonpolar organic solvents. See, for example, "Fluorination of Polyolefin Container During Blow Molding to Reduce Solvent Permeation, "Plastics and Rubber Processing, (March 1979), pages 10-16.
Another commercially available process for enhancing the barrier properties of blow-molded containers, known as the Dow sulfonation process, employs post-treatment of the container with a mixture of sulfur trioxide and nitrogen or dry air. This step is followed by treatment with ammonia and a dry diluent gas. This technique is discussed in the article, "Industrial Blow Molding: The Sleeping Giant Stirs," Modern Plastics, (November, 1977), pages 34-37.
It has also been proposed to improve the barrier properties of polyolefins by blending, for example, with polyamides. See, Plastics World, (July, 1983) at 33, 87.
A method for evaluation of solvent retention of surface fluorinated thermoplastics, as disclosed by Dixon et al. '284, is measuring gross loss of weight from toluene-containing bottles, kept at 100.degree. F., for various periods of time. Another method comprises filling treated containers with motor oil, placing the filled bottles on filter paper and determining the time required for the oil to penetrate through the container to the filter paper.
Quality control in the manufacture of thermoplastic containers is accordingly limited by the lack of a rapid, inexpensive method to determine the efficacy of the surface treatment or polymer modification in decreasing solvent loss by permeation through the walls of the container. Any practically useful test for barrier properties must be rapid, permitting detection of variations in product properties very fast, so that immediate corrective action can be taken.
The tests described above are typical of methods which directly measure permeability of solvent through the walls of the container. Indirect methods, which measure properties other than permeability, but which can be related to permeability, can also be used.
One direct method for determining permeability is the pressure-accelerated permability method, in which a sample is cut from a treated container and mounted in a high pressure test cell. Liquid or gas is forced through the wall of the container by diffusion under high pressure. The material diffusing through the wall can be detected by physical or chemical means. This method is less than optimum because it is a destructive quality control test and because days or weeks may be required for determining the permeability of a particular sample.
Attempts have been made to measure permeability directly by exposing the inner surface of barrier-coated thermoplastic material, or a sample cut from the product, to a solution of an intensely colored or fluorescent dye, removing the solution after a preset period of time and determining the degree and depth of dye penetration into the walls of the product visually or instrumentally. This method is limited to products free of interfering dark colored and/or opaque pigments. correlate with those of colored or fluorescent dyes employed. Even when used for evaluation of specimens free from interfering additives, these tests are not highly reliable.
Available indirect tests for effectiveness of surface treatment include chemical or physical detection of the active component in the barrier layer, for example, fluorine in the AIROPAK.RTM. system. When fluorine is used as the treating material, X-ray fluorescence, electron spectroscopy for chemical analysis (ESCA) or combustion, followed by chemical analysis, can be used. The ESCA technique employs low energy X-rays, which dislodge core electrons of molecules near the surface of the specimen being analyzed, and therefore permits specific analysis for elements at the surface of the sample.
Other techniques for determining surface properties of plastic materials include measurements of contact angle or total reflectance. These methods are often unreliable. Multiple internal reflectance (MIR), in which infrared data are analyzed by Fourier transform analysis, is considered more reliable, but is too complex to be employed for routine determination of surface properties.
Methods which test only small portions of the treated surface frequently fail to detect containers with unacceptable barrier properties, because a given surface may have been treated in a non-uniform fashion or because the polymer blend is not homogeneous. Optical and physical property determinations are also highly sensitive to contamination and, in some cases, difficult to correlate with barrier properties.
Evaluation of the permeability of a material toward carbon dioxide is often done by measuring decrease in pressure as a function of time. A commonly-utilized test requires study of 24 bottles for 24 weeks. Pressure loss through a film, employed in a gas permeation cell; analysis of organic vapors by gas chromatography; employing a Linde cell; and use of a cell in combination with an Oxtran.RTM. oxygen analyzer (sold by Modern Controls) are alternative methods for evaluating permeability of films to gases and/or liquids. None of the foregoing methods permits the rapid accumulation of data, required for controlling a production line in a container plant.
Sorption of bromine from aqueous solutions by polyethylene has been studied by Allen et al., Aust. J. Appl. Sci., vol. 12 (1961), pages 42-55. The reported mechanism of permeation of permanent gases through polyethylene is a combination of diffusion and solution.
It has been proposed by Morrisey et al. (U.S. Pat. No. 3,649,472) to test porosity of an electoplated article by determination of an electrolytic property, specifically corrosion potential.
Corrosion, measured as a function of time, is used by Just (U.S. Pat. No. 3,303,109) as a measure of diffusion profiles of semiconductor bodies.
Penetration of paper by a liquid has been assayed by a light-sensitive technique, as proposed by Berry et al. in U.S. Pat. No. 3,512,003.
The use of an electrical-colorimetric method to test the surface properties of a metal sample has been proposed by Alburger (U.S. Pat. No. 3,530,045).
It is accordingly the object of this invention to provide a rapid, economical, quantitative testing method, which correlates readily with surface permeability of thermoplastic containers, films or specimens.